D-lactate dehydrogenase is an FAD-containing enzyme in the respiratory chain of E. coli bacteria. D-LDH has been used as a model to study the electron-transfer process of membrane proteins. Because of the cofactor, FAD is capable of transferring either one or two electrons; an intermediate of the one-electron-reduced semiquinone may be formed. We have observed the ESR signal of semiquinone of D-LDH. With the use of the loop-gap resonator and stop-flow ESR technique, we hope to study the role of D-LDH semiquinone during rapid enzymatic reactions.